This invention relates to a magnetic recording medium for carrying out information recording and reproducing operations and, in particular, to a magnetic disk used in a magnetic recording/reproducing apparatus such as a HDD (Hard Disk Drive).
A magnetic recording medium used in a magnetic recording/reproducing apparatus such as a HDD (Hard Disk Drive) is increased in coercive force in order to meet the recent demand for a higher recording density. On the other hand, in order to achieve a higher recording density, the decrease in medium noise is indispensable. In the past, the decrease in medium noise is achieved by reducing Mr·t (the product of residual magnetization and film thickness) or by improving a film structure or a film material of the magnetic recording medium, with the help of uninterrupted improvement in output characteristics of a magnetic head.
It is noted here that Mr represents residual magnetization and t represents the thickness of a magnetic layer.
The reduction in Mr·t is extremely effective in improving R/W (read/write) characteristics but simultaneously causes a problem of thermal decay. Generally, the decrease in Mr·t is designed by reducing the thickness t of the magnetic layer. The reduction in thickness t brings about miniaturization of the grain size of the magnetic layer, resulting in reduction of the medium noise. However, with the increase in recording density, miniaturized crystal grains no longer have a coercive force (Hc) sufficient to hold recorded magnetization as a recorded signal. As a result, the thermal decay, i.e., the phenomenon that the recorded signal is attenuated, becomes remarkable.
As a magnetic recording medium capable of preventing the phenomenon (thermal decay) that the recorded signal is attenuated and of obtaining a high coercive force, attention is attracted to a magnetic recording medium comprising a substrate having a texture formed on the substrate along a circumferential direction thereof to thereby improve magnetic anisotropy in the circumferential direction. Generally, the magnetic anisotropy is represented by an OR (Oriented Ratio) which is a ratio of a magnetic characteristic in circumferential direction to another magnetic characteristic in the radial direction. Generally, excellent thermal decay resistance is achieved by a greater OR. Herein, the magnetic characteristic may be a coercive force (Hc) or the product of residual magnetization and film thickness (Mr·t).
A magnetic recording medium having magnetic anisotropy is proposed, for example, in U.S. Pat. No. 4,735,840. The magnetic recording medium comprises a substrate with a circumferential texture formed on its surface and an underlayer, a magnetic layer, a protective layer, and a lubrication layer successively formed on the substrate. In this technique, use may be made of an aluminum substrate, a glass substrate, or a polycarbonate substrate having an NiP alloy film.
In recent years, the demand for a higher recording density and a lower cost of the magnetic recording medium becomes more and more strict. The substrate with the NiP alloy film requires a complicated process and is an inhibiting factor against the reduction in cost.
The glass substrate high in rigidity and flatness allows a low flying height of a magnetic head. Therefore, the glass substrate recently attracts attention as a substrate suitable for an increase in recording density. Therefore, it is predicted that the glass substrate with a circumferential texture directly formed on its surface without forming the NiP alloy film is desired. However, a magnetic recording medium provided with the circumferential texture directly formed on the surface of the glass substrate has an oriented ratio (OR) which hardly satisfies a desired level. Thus, such glass substrate is insufficient to meet the recent demand for a higher recording density.
According to the research by the present inventors, the above-mentioned oriented ratio (OR) is as small as about 1.03 when the underlayer, the magnetic layer, the protective layer, and the lubrication layer are successively formed on the glass substrate with the circumferential texture directly formed on its surface. In particular, it has been found out that the magnetic recording medium having a high coercive force (for example, about 2500 oersted or more) is difficult to be given a high oriented ratio (OR) of, for example, 1.1 or more.